Us old aero types arn't the stick in the muds you might think, we have thought of the wing tip roll device and other things many times. If you don't see things used on airplanes today they have probably proven to be less than desireable. After all it has been 100 years and many wars to make the technology mature.

Many years ago I worked up some studies on this subject. I have since lost them but the basic result is that if the aileron chord is much more than 30 percent of the wing chord the increase in effectiveness is not worth the weight to accomplish it. The lightest structure was one that did not have the pivot in the wing tip. Most of the time it is possible to obtain all necessary rolling moment with conventional ailerons - any comanded roll that you can't control is pretty well wasted anyway.

Those are basically the reasons you don't see these kinds of tip devices used on airplanes of any kind today.

Just because the F-15 uses a differential tail does not mean that the differential tail is better for our model use - just that it was a good addition to the ailerons for the F-15 over it's required flight envelope. The all moving tail is there because there are flight conditions (mach number, angle of attack, etc.) at which the ailerons are not effective and yet roll is still required.

For instance think of the F-15 flying at 25 degrees angle of attack. The tail is deflected about that angle leading edge down. (I don't remember the exact numbers, getting old) At this angle of attack the ailerons have very little roll power but the horizontal is basically parallel to the airflow. This gives a roll moment capability when nothing else is working.

There are high mach numbers (dynamic pressures) where the flexibility of the wing makes the aileron less effective. Here the differential tail can be useful.